Separating retorted shale from recycled heat-carrying pellets

ABSTRACT

Hot pellets in a size range between about 0.06 and 0.3 inch characterized primarily by their effective surface area, quantity, size, density, and elutriation velocity are cycled to a retort zone to mix with and retort crushed oil shale, thereby producing gas and oil products, a combustible deposition on the pellets, and spent shale. The main source of heat for retorting is derived from controlled burning of the deposition on the pellets. After retorting and prior to burning the deposition, at least 95 percent by weight of the spent shale is separated from the pellets in a way which recovers at least 99 percent by weight of the pellets for reuse in the process. Separation and recovery of the pellets is accomplished by a multiple stage combination of gas elutriation at increasing elutriation velocities and appropriate screening of the elutriated particles when pellets are present therein.

United States Patent [191 Abdul-Rahman [111 3,803,021 |4s| Apr. 9, 1974 SEPARATING RETORTED SHALE FROM RECYCLED HEAT-CARRYING PELLETS [75] lnventor: Yahia A. K. Abdul-Rahman, Plano,

Tex.

[73] Assignees Atlantic Richfield Company, Los Angeles, Calif.

[22] Filed: Dec. 26, 1972 [21] Appl. N0.: 318,190

[52] US. Cl. 208/11 [51] Int. Cl Cl0b 53/06 [58] Field of Search 208/11 [56] References Cited UNITED STATES PATENTS 3,020,227 2/1962 Nevens et al. 208/11 3,164,541 1/1965 Linden et al 3,442,789 5/1969 Zimmerman 208/11 Primary E\'aminerCurtis 'R. Davis [5 7 ABSTRACT l-lot pellets in a size range between about 0.06 and 0.3 inch characterized primarily by their effective surface area, quantity, size, density, and elutriation velocity are cycled to a retort zone to mix with and retort crushed oil shale, thereby producing gas and oil products, a combustible deposition on the pellets, and spent shale. The main source of heat for retorting is derived from controlled burning of the deposition on the pellets. After retorting and prior to burning the deposition, at least 95 percent by weight of the spent shale is separated from the pellets in a way which recovers at least 99 percent by weight of the pellets for reuse in the process. Separation and recovery of the pellets is accomplished by a multiple stage combination of gas elutriation at increasing elutriation velocities and appropriate screening of the elutriated-particles when pellets are present therein.

24 Claims, 1 Drawing Figure SEPARATING RETORTED SHALE FROM RECYCLED HEAT-CARRYING PELLETS BACKGROUND OF THE INVENTION This invention relates to a retorting process wherein heat-carrying pellets are used to retort crushed raw oil shale and must be separated and recovered from the retorted spent shale particles for reheating and recycle through the process.

As a preliminary stage in the production of petroleum oils and gases, the solid carbonaceous organic solid matter or kerogen in oil shale is pyrolyzed or retorted. Retorting denotes thermal conversion of kerogen or organic matter to oil vapors and gas, thereby leaving solid particulate spent shale and includes separation of the oil vapors and gas from the spent shale.

The spent shale contains residual carbonaceous organic matter and matrix mineral matter.

When the kerogen is retorted, a normally gaseous fraction, a normally liquefiable vaporous fraction, and a combustible organic residue are formed. The product distribution between gas, liquid, and residue is important and relates to the distribution of the various boiling point fractions in the liquid product. Copending applications Ser. Nos. 284,288, filed Aug. 28, 1972; 285,732, filed Sept. 1, 1972; 304,074, filed Nov. 6, 1972; and 308,136, filed Nov. 20, 1972, which are owned by a common assignee and are incorporated herein, provide processes for retorting 'oil shale using special pellets in a way which causes a combustible organic carbon deposition to be formed on the pellets during retorting of oil shale and improves the recovery of useful components and liquid product distribution. Some or all of this combustible deposition is burned in a pellet deposition burning zone to heat and reheat the pellets. In such processes, it is important that most of the spent shale be separated from the pellets prior to burning and that substantially all of the pellets of a significant size be recycled through the process.

The special pellets are comprised of particulate or divided solid heat carriers of a size and porous nature designed to exhibit an appropriately high surface area and appropriate heat transfer and deposition burning characteristics. The size of the pellets is such that it is difficult to separate the pellets from the spent shale by elutriation without losing an excessive amount of pellets or without causing slugging or bridging. In addition, this type of pellet undergoes size attrition or reduction as it is cycled through the process. Consequently, even if a narrow large size range of pellets were initially used, the size range of the pellets would soon overlap the size of some of the larger size spent shale, thereby increasing the difficulty of properly separating the pellets from the spent shale.

This invention provides a method of retorting using porous pellets and of separating at least 95 percent by weight of the spent shale while recovering at least 99 percent by weight of the pellets within a size range between about 0.06 inch and 0.3 inch. The separation and recovery steps involve a multi-stage combination of elutriation and screening.

SUMMARY OF THE INVENTION Mined oil shale which contains solid carbonaceous organic matter and other mineral matter and which has been crushed and may have been preheated is retorted in a retort zone with the hot heat-carrying pellets at a temperature and in an amount sufficient to provide at least 50 percent of the sensible heat required to retort the oil shale. Retorting oil shale produces gas and oil products, which are recovered, and particulate spent shale. In the retorting process, a combustible carboncontaining deposition is deposited on the pellets.

The pellets are comprised of porous particulate or divided solid heat carriers in a size range between about 0.06 inch or larger and 0.3 inch and whose physical properties and characteristics, especially surface area, temperature, amount, size, shape, and particle weight or density, coact with process variables to accomplish the objectives and advantages of the process. The size, density, and shape of the pellets are selected such that for a particular size, the elutriation velocity or terminal falling velocity of the spent shale is significantly lower than the same size pellet.

After the oil shale is retorted, the process of this invention uses the aforementioned property of the pellets and a multi-stage combination of elutriation steps and screening to aid in separating at least percent by weight of the spent shale from the pellets and in recovering for reuse in the process at least 99 percent by weight of the pellets. Briefly, a mixture of spent shale and pellets from the retort either with or without first being passed through an initial screening is passed to a low velocity gas elutriation zone to separate the smaller size spent shale but none of the pellets, that is, pellets between the smallest sized pellets or about 0.06inch, whichever is larger, and 0.3 inch. The remaining mixture of pellets and spent shale is subsequently subjected to gas elutriation using a higher elutriation velocity to elutriate pellets and spent shale of the same elutriation velocity, but as previously shown, of different size with the spent shale being the larger of the two. The elutriated product is then screened to pass the smaller pellets to a pellet recycle system and to retain and discharge the larger spent shale to disposal. The remaining mixture of pellets and spent shale is then subjected to another gas elutriation stage at a higher elutriation velocity than the preceding elutriation stage and the elutriated product being screened as before. Generally, only three stages of elutriation will be required, but if needed, the elutriation-screening steps can be repeated until the prescribed objectives are achieved. In addition to efficiently accomplishing the desired objectives, the separation-retention steps permit the use-of lower capacity screening and lower gas volumes wherein the gas from one stage can be used in the next stage and eventually for lift of the pellets to a pellet deposition burning zone.

After separation and retention, the pellets are lifted to a pellet deposition burning zone wherein the pellets are reheated for recycle to the retort zone.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a partly schematical, partly diagrammatical flow illustration of a system for carrying out a preferred sequence of the process of this invention.

DETAILED DESCRIPTION OF THE INVENTION A process for retorting crushed oil shale containing a carbonaceous organic matter, commonly called kerogen, and other mineral matter with hot pellets which are later separated from the spent retorted shale and recycled is described having reference to the drawing.

by way of shale inlet line 11 into retort zone 12. At the same time, heat-carrying pellets substantially hotter than the shale feed are fed by gravity or other mechanical means to the retort zone by way of pellet inlet pipe 12. The pellets and shale feedstock could be fed to the retort zone by way of a common retort zone inlet.

Crushing of the raw mined shale expedites more uniform contact and heat transfer between the shale feedstock and hot pellets. In normal practice, the degree of crushing is simply dictated by an economic balance between the cost of crushing and the advantages to be gained by crushing when retorting the kerogen from the shale. Generally, the shale feedstock is crushed to about one-half inch and no particular care is taken to produce or restrict production of finer material.

The crushed oil shale may or may not be preheated by direct or indirect heat from any source including indirect heat exchange with pellets or flue gases generated during this retorting process. If the shale feedstock is preheated, the temperature of the feedstock will usually not exceed 600F. The shale feedstock will usually be fed by way of a metered weight controller system, for reasons hereinafter made apparent, and which may include a preheat and/or gas lift system. The preferred system for preheating the raw shale is to lift the shale in lift pipes with the hot flue gases generated in the combustion phase of the process.

The hot heat-carrying pellets are at a temperature ranging between l,0OF and l,500F which is about 100 to 500F higher than the designed retort temperature within the retort zone. The most favorable practical temperature range depends on other process variables. The quantity of pellet heat carriers is controlled so that the pellet-to-shale feedstock ratio on a weight basis is between 1 and 3. This ratio is such that the sensible heat in the pellets is sufficient to provide at least 50 percent of the heat required to heat the shale feedstock from its retort zone feed temperature to the designed retort temperature. The feedstock feed temperature is the temperature of the oil shale after preheating, that is the temperature of the shale upon entry into the retort. The average retort temperature ranges between about 850F and 1,200F, depending on the nature of the shale feedstock, the pellet-to-shale ratio, the productdistribution desired, heat losses, and the like.

As used herein, the term pellets refers to subdivided or particulateheat-carrying bodies having a minimum relatively high surface area of square meters per gram and higher, a size range between about 0.06 inch and 0.3 inch including a narrower range therewithin, and a shape and density or particle weight such that for a particular size the elutriation velocity of a pellet of that size is significantly (measurably and usefully) higher than the elutriation velocity of most of the particles of spent shale of that size. Preferably, the average surface of the pellets will be between 10 and 150 square meters per gram. The surface area is the average efiective of the pellets as they enter the pyrolysis zone. The surface area may be determined by the conventional nitrogen absorption method. The pellets may be cylindrical shape, approximately oval or spherical shape, or purely spherical shape. The much preferred pellets have a sphericity factor of at least 0.9 which shape gives the highest particle weight and is particularly useful in separating the pellets by elutriation and screening from spent shale produced in the retort zone. The sphericity factor is the external or geometric surface area of a sphere having the same volume as the pellet divided by the external surface area of the pellet.

The pellets are made up of materials such as alumina or silica alumina, which are not consumed in the process and which are subdivided or particulate matter having significantly high internal surface area. The pellets are sufficiently wear or breakage resistant and heat resistant to maintain enough of their physical characteristics under the conditions employed in the process to satisfy the requirements'of the process, to affect retorting of the oil shale, and to permit controlled burning of a carbon-containing deposition formed on the pellets during the process. More specifically, the pellets do not disintegrate or decompose, melt or fuse, or undergo excessive surface area reduction at the temperatures encountered during such burning and the thermal stresses inherent in the process. Unfortunately, the pellets do, of course, undergo gradual wear or size reduction. As will be shown, this makes it difficult to accomplish the necessary pellet separation and recovery which is especially critical to this particular type of pellet oil shale retorting process. The original or fresh pellets will usually be between 0.15 and 0.3 inch with a range between 0.2 and 0.3 being initially sought. After wear or attrition, the size range will increase and great effort is made to retain and recover all pellets above about 0.06 inch or plus 14 U.S. Sieve Series Screen size. Finer grain pellet-like particles which were once part of the pellets may be present, but no special effort is made to retain these finer grains and a large percentage of substantially smaller finer grained pellets is undesirable.

The retort zone is any sort of retort which causes intimate contact or mixing of the crushed oilshale and pellets. The preferred retort is any sort of horizontal or inclined retorting drum that causes the oil shale and pellets to undergo a tumbling action. This sort of retort is herein referred to as a rotating retort zone. This type of retort zone is quite flexible over a wide range of conditions and has the advantages of causing rapid solidtosolid heat exchange between the pellets and shale feedstock thereby flashing and pyrolyzing the oil and gas vapors from the shale in a way which allows the vapors to separate from the solids without passing up through a long bed of solids and which minimizes dilution of the product vapors by extraneous undesirable retorting gases; of allowing for a high shale throughput rate at high yields for a given retort volume, of providing for greater control over residence time, of aiding in preventing overcoking and agglomeration of the pellets and shale; of facilitating formation of a more uniform controlled amount of combustible carbon-containing deposition on the surface area of the pellets; and of causing flow of the pellets and shale through the retort zone in a manner which aids in eventual separation of the pellets from the spent shale.

The retorting process is carried out in concurrent or parallel flow fashion with the hot pellets and the raw shale feedstock being fed into the same end of the retort. The retort zone may be maintained under any pressure which does not hamper efficient operation of the retort, interfere with production of valuable retort vapors, or cause excessive deposition of residue on the pellets. Generally, pressurization of the pyrolysis or retort zone causes considerable difficulties, especially if a rotating retort zone is used. The pressure employed is, therefore, generally the autogenous pressure.

In the retort zone, the hotter pellets arid cooler crushed shale feedstock are admixed and intimately contacted almost immediately upon being charged into the retort zone. The shale particles are rapidly heated by sensible heat transfer from the pellets to the shale. Any water in the shale is distilled and the kerogen or carbonaceous matter in the shale is decomposed, distilled, and cracked into gaseous and condensable oil fractions, thereby forming a valuable vaporous effluent including gas, oil vapors, and superheated steam. Pyrolysis and vaporization of the carbonaceous matter in the oil shale leaves a particulate spent shale in the form of the spent mineral matrix matter of the oil shale and relatively small amount of unvaporized or coked organic carbon-containing material.

As the aforementioned vaporous effluents are formed, a combustible carbon-containing deposition or residue will be formed or deposited on the pellets if the effective surface area of the pellets has not already been covered with all of the deposition that it can sustain. The total amount of deposition formed or deposited on the pellets upon one passage through the processis sufficient upon combustion to provide at least 50 percent of the heat required to reheat the pellets. The amount of combustible deposition deposited on the pellets during the retorting stage is on an average less than 1.5 percent by weight of the pellets, and the preferred range is between 0.8 and 1.5 percent. The pellet surface area, size, and amount coacts with other process conditions to accomplish the desired amount of combustible deposition and product distribution. If the surface area of the pellets is less than square meters per gram, either too little total deposition will be formed or the burning of the deposition will not be sufficient to provide a major portion of the heat required to heat the pellets to the desired temperature and to carry out the retorting phase of this process. This would necessitate the use of supplementary fuels which is undesirable.

The mixture of pellets and shale moves through the retort zone toward retort exit 14 and the gaseous and vaporous effluents containing the desired hydrocarbon values separate from the mixture. The residence time for the pellets required to effect retorting and deposition of the pellet deposition is on the order of about 3 to about 20 minutes with residence times of less than 12 minutes for the pellets being preferred. Ths shale residence time depends on its flow or movement characteristics and since the shale is not uniform in size and shape, the shale residence time varies.

The mixture of pellets and spent shale exits from retort zone 13 at a temperature between 800F and 1,050F by way of retort exit 14 into recovery chamber 15 where the gaseous and vaporous products resulting from retorting the oil shale collect overhead and rapidly pass to overhead retort products line 16 at an exit temperature between about 750F and l,O50F. The product vapors are usually subjected to hot dust separation and the dust thus collected may be combined and handled with other spent shale. Hot dust or fines separation may be accomplished by hot gas cyclones, quenching'and washing, agglomeration with sludge or a separately condensed heavy product fraction, centrifuging, filtration, or the like.

The retort zone particulate mixture of pellets and spent shale are passed through a pellet separation and recovery zone wherein on an average or on a reasonable time basis at least 95 percent by weight of the spent shale is separated and on an average at least 99 weight percent and preferably at least 99.5 percent of the pellets are recovered for reuse in the process. As

shown, the mixture is first passed through optional revolving screen or trommel 17 extending into product recovery chamber 15 and which has openings or apertures sized to pass the pellets and spent shale of the same or smaller size than the pellets and screen out any spent shale and agglomerates larger than the pellets. Most of the spent shale and the pellets flow through the openings in trommel l7 and drop to the bottom of recovery chamber 15 to exit via exit line 18. Any spent shale, mineral matter, and agglomerates too large to pass through the openings in the trommel pass outward through exit 19 to spent shale disposal. This prescreening or initial separation of spent shale larger than the pellets is optimal and may also be delayed until a later or final stage of the separation system. The rotating retort makes initial screening easier.

The spent shale and pellets in recovery chamber 15 are discharged via exit line 18 at a temperature between about 0F and l,O50F where these particulate solids are passed or conducted by gravity or other means of conveyance to a subsequent phase of the separation and recovery zone involving a multi-stage combined gas elutriation-screening pellet separation and recovery system in which on an aggregate at least 60 percent by weight of the spent shale of the same size as the pellets is separated from the pellets leaving the retort and at least 99 percent by weight of the plus 14 US. Sieve Series Screen size pellets, that is, approximately 0.06 inch and larger, are recovered for recycle in g the process. The multi-stage gas elutriationscreening system may be conducted in batchwise fashion be sequentially changing operating conditions, or a combination of batch and separate units, or three or more separate units or stages. Preferably, as shown, the multiple stages will use separate elutriators and screening units for each appropriate stage. The internal operation and design of gas elutriators is well'known; however, it may be noted that, in addition to the properties of the particles, the elutriation velocity of particular particles depends on the elutriator design including such factors as free board height, bed height and weight, gas type and velocity, column diameter and cross-sectional area, and transport disengaging height. In this process, it is highly desirable that the elutriating gas be maintained at a temperature above 750F, and it is essential that elutriation be accomplished in a way which retains the desired amount of combustible deposition on the pellets; consequently, the elutriating gas must be a noncombusting gas, that is, a gas that will not burn the combustible deposition on the pellets.

The following description of the multi-stage combined elutriation and screening system can best be understood by first considering the following two tables illustrating particle size distributions of the pellets and spent shale and a set of average elutriation velocities for narrow cuts of spent shale and pellets:

TABLE 1 ILLUSTRATIVE PARTICLE SIZE DISTRIBUTIONS (Percent by Weight) ILLUSTRATIVE ELUTRIATION VELOCITIES AND SIZE Particle Size Elutriation Velocity, fps

(inch) Pellets Spent Shale 0.06 21 l8.5 0.10 3| 27 0. X5 38 34 0.20 42 37.5 0.25 45 40 Table 2 illustrates the fact the elutriation velocity of spent shale of a particular size is less than the elutriation velocity of the same size pellets. This property is taken advantage of in the elutriation and screening stages. I

The mixture of pellets and spent shale passes from the retort zone through the recovery chamber via line 18 into first elutriation zone 20 where the mixture is subjected to gas elutriation with a noncombustion supporting gas entering the elutriation zone by way of inlet 21. Preferably, the gas will be heated. The gas may be hot flue gas or steam or any other noncombustion supporting gas. The elutriation velocity of the gas is below the elutriation velocity of the smallest size pellets which by definition cannot be below about 0.06 inch pellets (plus l4 screen size). Thus, in the first elutriation stage, practically no pellets are elutriated or carried out of the elutriation zone by way of overhead line 22 by the elutriating gas. But the elutriating gas velocity is at least high enough to elutriate spent shale that is smaller than the smallest size pellets in the mixture and is, therefore, at least high enough to elutriatespent shale that is 0.055 inch and smaller. For illustrationpurposes only, Table 2 shows that an.elutriation velocity of -l8.5 feet per second could be used, but that the 0.06 inch pellets will not be elutriated at this velocity. If the smallest'size pellets were larger than 0.06 inch, a higher elutriation velocity could be used. The elutriated spent shale is separated as an overhead stream from first elutriation zone 20, thereby leaving a mixture of remaining pellets and spent shale. It is highly desirable that elutriation in the first stage be conducted until the percentage'by weight of spent shale smaller than the smallest size pellets in this mixture be less than 3 percent by weight. This will partly depend on the capacity of the first elutriation stage. As previously stated, at least 95 percent by weight of the spent shale will be separated in the overall separation and pellet recovery process. As will hereinafter be made apparent, fine spent shale will tend to follow the pellets and an optional final elutriation low velocity elutriation zone can be used to remove finer shale carried forward. But if the first stage has adequate capacity, it is preferred to remove the finer spent shale in the first stage.

The spent shale is separated from the elutriating gas in first overhead separator 23, for example, a cyclone, scrubber, or the like, to be disposed of by way of line 24, and the clean elutriating gas may be passed through gas exit-inlet line 25 to, as shown, the next or second elutriation zone 26 or, as not shown, to be recycled into the first elutriation zone or to a pellet lift system.

The mixture of remaining pellets and spent shale leaves the first elutriation zone as a lower stream through lower exit line 27 and is passed to another or second elutriation zone where the mixture is subjected to gas elutriation with gas entering line 25, which may be the same gas as was used in the first zone, or a fresh gas, or a combination of the two with or without additional heating. The elutriation velocity in the second zone is higher than the elutriation velocity of the preceding or first elutriation zone and is at least high enough to elutriate the smallest size pellets in the mixture. As indicated by Table 2, when this velocity is used, spent shale particles larger than the elutriated pelletswill also be elutriated and carried out in overhead stream 28 with the elutriated pellets. This leaves remaining particulated solids that are a mixture of pellets and spent shale which now contains pellets of an average size larger than the average size of the pellets prior to elutriation because the smaller pellets have been removed.

The overhead stream of elutriated pellets and spent shale is separated from the elutriating gas in a second overhead separator 29, and the separated elutriating gas may be passed through gas exit-inlet line 30, as shown, to the next or third elutriation zone 31 or, as not shown, to be recycled to the first or second elutriation zones or to a pellet lift system.

The separated mixture of elutriated pelletsand spent shale is passed through line 32 to screening unit 33 which is any sort of efficient screening or trommel-like separating system operating to retain the spent shale larger than the pellets and to pass andallow recovery velocity of the spent shale is less than the elutriation velocity of the pellets. The larger size retained spent shale is passed throughexit 34 to disposal. The pellets are passed through exit line 35 and recovered for eventual passage to pellet return line 36.

The remaining particulated solids containing pellets and spent shale is passed from second elutriation zone 26 leaving the zone as a lower stream through line 37 to elutriation zone 31 where, as before, the mixture is subjected to gas elutriation with gas entering line 30 which may be the same gas as was used in the second or first zone, or a fresh gas, or acombination of these gases with or without additional heating. The elutriation velocity in the second zone is higher than the elutriation velocity of the preceding or second elutriation zone and is at least high enough to elutriate the smallest size pellets in the mixture which size 'is larger than the smallest size pellets in the second elutriation zone. As previously indicated, this in turn causes spent shale larger than the pellets in the third zone to be elutriated and carried out in overhead stream 38 with the elutriated pellets. This leaves remaining particulated solids which may be essentially only pellets or a mixture of pellets and spent shale. The remaining pellets will be of an average size larger than the average size of the pellets prior to elutriation because a group of smaller pellets has been removed by elutriation.

As before, the overhead stream of elutriated pellets and spent shale is separated from the elutriating gas in third overhead separator 39. The separated elutriation gas may be passed through gas line 41 to a subsequent elutriation zone if any is needed, or to be recycled to the first or second elutriation, or as shown, partly to pellet lift system and partly to optional elutriation zone 43.

The separated mixture of elutriated pellets and spent shale is passed through line 45 to screening unit 46 operated to retain the spent shale larger than the pellets and to pass and allow recovery of the pellets and any small size spent shale of equal or smaller size than the pellets. The larger size retained spent shale is passed through exit 47 to disposal. The pellets are passed through exit line 48 and recovered for eventual passage to pellet return line 36.

If the remaining particulated solids from the third elutriation zone contain a substantial amount of spent shale, the remaining mixture will be passed to a subsequent elutriation-screening stage operating at higher conditions to further separate the pellets and spent shale until on an average or reasonable aggregate time basis at least 60 percent, and preferably at least 70 percent, by weight of the spent shale from the retort zone having the same size as the pellets is separated from the pellets from the retort zone. Generally, however, only one initial elutriation stage and two elutriationscreening stages will be needed to separate the spent shale of the same size as the pellets.

The pellets remaining after the last elutriation stage are passed by way of line 49 from the third elutriation zone and recovered for eventual passage to pellet return line 36.

As shown, the recovered separated pellets in lines 35, 48 and 49, collected in line 50, contain on an average at least 99 percent by weight of the pellets leaving the retort zone through line 18. The pellets may still contain time spent shale not previously separated and passing through screening units 33 and 46. If so, the collected pellets may be passed to optional final elutriation zone 43 where the pellets may be subjected to low velocity gas elutriation at a velocity below the elutriation velocity of the smallest pellets to remove the fine spent shale without removing any of the pellets.

As before, the elutriated spent shale will exit overhead through line 51 to separator 53 where the separated spent shale in line 55 is sent to disposal and the gas in line 57 may, as shown, be sent to pellet lift system 59. The pellets exit the optional elutriation zone via pellet return line 36 to pellet lift system 59, where the pellets are lifted preferably to an elevation which allows gravity feed to retort zone 12 by way of lift line 61 to pellet deposition burning zone 62 which, as shown, has surge hopper 63 for collecting the lifted pellets and leveling out fluctuations and from which the pellets fall into pellet deposition burning zone 64. While any conveying and lifting system holding heat losses to a reasonable value may be used, it is preferred, as shown, that the pellet lifting system be a pneumatic or gas conveying system which will operate in the conventional manner to lift the pellets to the pellet deposition burning zone. The lift gas enters the lift system via line 41 at a velocity sufficient to lift the pellets.

As mentioned previously, the pellets bear a combustible deposition which was absorbed or deposited during the process. This combustible deposition is burned in combustion or pellet deposition burning zone 64 to provide at least 5 0 percent or more of the heat required to reheat the pellets to the temperature required to effect retorting of the shale. The combustible deposition is burned in a manner similar to the way that cracking catalysts particles are regenerated and which is controlled to avoid excessive heating of the pellets which would excessively reduce the effective surface area of the pellets to less than 10 square meters per gram. A progressive bed burner with a gas flow of about 1 to 2 feet per second is preferred. A combustion supporting gas, for example air, a mixture of air and fuel gas generated in the process, or flue gas with the desired amount of free oxygen, is blown into the pellet deposition burning zone at a temperature at which the deposition on the pellets is ignited by way of combustion gas inlet 65. Steam may also be used to control burning provided that the steam does not excessively reduce surface area of the pellets. The combustion supporting gas may be preheated in heaters 66 by burning some of the gases produced in the process to reheat the pellets to the minimum ignition temperature. The quantity of combustion supporting gas, e.g., about 10 to 15 pounds of air per pound of deposition, affects the total amount of deposition burned and the heat generated by such burning and, in turn, the temperature of the pellets. The bulk density of the pellets is about 40 to 50 pounds per cubic foot, and the specific heat of the pellets varies between about 0.2 and 0.3 British Thermal Units per pound per degree Farenheit. The gross heating value of the carbon-containing deposition is estimated to be about 15,000 to 18,000 BTU per pound. The amounts of carbon dioxide and carbon monoxide produced in the flue gases created by burning the pellet deposition indicate the amount of combustion supporting gas required or used and the amount of carbon-containing deposition not burned. Generally, it is desirable to attempt to free the pellets of deposition. Other factors taken into consideration during burning of the pellet deposition are the pellet porosity, density, and size, the burner chamber size and pellet bed size, residence burning time, the desired temperature for the pellets, heat losses and inputs, the pellet and shale feed rates to the retort zone and the like. The residence burning time will usually be rather long and up to about 30 to 40 minutes. Combustion of the deposition should be controlled in a manner which does not heat the pellets to above l,500F. The hot flue gases generated in the pellet deposition burning zone may be removed by burning zone exit line 67 and used to preheat cool raw shale feedstock or for heat transfer to any other phase or part of the shale operation. For example, this stream could be fed to a carbon monoxide boiler and the heat available from the boiler could be used for processing product vapors or to drive turbines. Of course, additional fuel material or gases may be used to supplement burning of the pellet deposition if this is necessary.

The hot temperature burning conditions in the pellet deposition burning zone tend to cause spent shale particles carried into the zone to undergo size attrition or splintering, thereby helping to prevent buildup of unseparated spent shale as the pellets are cycled in the process. Otherwise, other means of attrition could be used.

A continuous stream of hot pellets having a temperature above 1,000"F and not exceeding l,500F is thereby produced. The hot pellets pass through a pellet deposition burning zone exit and eventually to retort inlet line 13 either by gravity and/or mechanical means to the retort zone. As previously indicated, the rate of passage of the pellets from the combustion zone will be metered or controlled in conventional manners to eventually provide the optimum pellet-to-oil shale feedstock ratio to the retort zone. The optimum ratio is governed by the pellet properties, the amount of deposition on the pellets as they enter the retort zone, the organic content of the raw oil shale, and the other process variables as previously described.

EXAMPLE In a retort zone, 11,000 tons per day of crushed oil shale preheated to 450F is retorted with 22,000 tons per day of hot spherical pellets having a surface area of 46 square meters per gram at a temperature of about 1,300F. The hot pellets provide the heat necessary to retort the preheated oil shale. About 1.24 weight percent of combustible deposition is deposited on the pellets in the retort. The gas and oil products produced during retorting are recovered and a mixture containing 22,000 tons per day of pellets and 8,910 tons per day of spent shale having the particle distribution of Table l is-passed .to a first elutriation zone and subjected to hot gas elutriation under conditions such that approximately 90 percent by weight or 8,019 tons per day of spent shale is removed for spent shale disposal. No pellets are carried over with the elutriated spent shale. A mixture of remaining pellets and spent shale containing 22,000 tons per day of pellets and 891 tons per day of spent shale is passed to a second elutriation zone and subjected to hot gas elutriation under conditions such that the 6 8 spent shale is elutriated with 70 percent efficiency. The elutriated mixture is screened producing 300 tons per day of spent shale which is sent to disposal and 220 tons per day of pellets which are recoveredand passed to a pellet lift system. The remaining mixture of pellets and spent shale is then passed to a third elutriation zone and subjected to hot gas elutriation under conditions such that the -4 6 spent shale is elutriated with 70 percent efficiency. The elutriated mixture is screened producing 300 tons per day of spent shale which is sent to disposal and 220 tons per day of pellets which are recovered and passed to a pellet lift system. The remaining mixture of pellets and spent shale is then passed to a third elutriation zone and subjected to hot gas elutriation under conditions such that the 4 6 spent shale is elutriated. The elutriated mixture is screened producing 203.4 tons per day of spent shale which is sent to disposaland passing 128.6 tons per day of spent shale and 2,860 tons per day of pellets which are recovered and passed to a pellet lift system. The remaining mixture of pellets and spent shale in the third elutriation zone contains 87.14 tons per day of spent shale and 18,920 tons per day of pellets which are also passed to the pellet lift system. Over all, 97.6 weight percent of the spent shale is separated and removed from the system and 75 percent or more of the spent shale of the same size as the pellets is separated and removed in the elutriation-screening system. The loss of pellets is very low and 99 weight percent and more of the pellets are recovered. The recovered pellets are then passed to a pellet deposition zone and reheated for return to the retort zone.

Although the retorting process is carried out in a manner to hold loss of pellets to a minimum, some pellets will be lost in the process and a relatively small quantity of pellets may be added continuously to maintain the pellet quantity.

Reasonable variations and modifications are practical within the scope of this disclosure without departing from the spirit and scope of the claims of this invention. For example, while the disclosure of this process and the variables have been limited to oil shale, the process concepts lend themselves readily to retorting any solid organic carbonaceous material containing hydrocarbon values which can be recovered by thermal vaporization of the solid carbonaceous material, such as, for example, coal, peat, and tar sands. By way of further example, while only a single train of units and stages have been described, it is to be understood that any stage or zone could be comprised of more than one stage or zone. I

The embodiments of the invention in which an exclusive property or privilege is claimedrare defined as follows: v 1. A method for retorting of crushed oil shale containing carbonaceous organic matter and mineral matter using heat-carrying pellets and for separating and recovering said pellets for reuse in the retorting method, which method comprises a. feeding crushed oil shale and pellets to a retort zone, said pellets being particulate heat carriers in a size range between approximately 0.06 inch and 0.3 inch and having a surface area of at least 10 .square meters per gram of pellets, said pellets being of a shape and particle weight such that for a particular size the gas elutriation velocity of a pellet of said particular size is significantly higher than the elutriation velocity of most spent shale particles of said particular size, said pellets being at a retort zone inlet temperature between 1,000F and 1,500F and in a quantity such that the ratio of said heat-carrying pellets to said crushed oil shale entering said retort zone on a weightbasis is between 1 and 3, said ratio also being such that the sensible heat in said pellets is sufficient to provide at least 50 percent of the heat required to heat said crushed oil shale from its retort zone feed temperature to a retort zone outlet temperature of between 800F and 1,150F;

b. retorting in said retort zone gas and oil products from said crushed oil shale, thereby forming a mixture of pellets and particulate spent shale;

c. recovering said gas and oil products generated by retorting said crushed oil shale;

d. passing said mixture of pellets and spent shale from said retort zone to a separation and pellet recovery zone and separating on an average at least percent by weight of said spent shale from said mixture and recovering on an average at least 99 percent by weight of the pellets in said mixture, said separating and recovering comprising l. passing a mixture of pellets and'spent shale to a elutriation zone and subjecting said mixture to gas elutriation with a noncombustion supporting gas at an elutriation velocity below the elutriation velocity of the smallest size pellets in said mixture but at least high enough to elutriate spent shale that is smaller than said smallest size pellets, the elutriated spent shale being separated as an overhead stream from said elutriation zone thereby leaving a mixture of remaining pellets and spent shale;

2. subjecting the mixture of remaining pellets and spent shale from the preceding gas elutriation stage to gas elutriation in an elutriation zone with a noncombustion gas at an elutriation velocity higher than the gas elutriation velocity of said preceding elutriation stage and at least high enough to elutriate the smallest size pellets in said mixture thereby also elutriating a portion of the spent shale larger than the elutriated pellets, the elutriated pellets and spent shale being separated as an overhead stream from said elutriation zone thereby leaving remaining particulate solids containing pellets of an average size larger than the average size of the pellets prior to said gas elutriation, screening said overhead stream to retain spent shale larger than the pellets in said overhead stream and to pass the pellets in said overhead stream, and recovering the screened pellets;

3. repeating step (2) until on an average at least 60 percent by weight of the spent shale from said retort zone having the same size as the pellets is separated from the pellets from said retort zone; and thereafter recovering the pellets remaining after the last elutriation stage, the total amount of pellets recovered in steps (2), (3), and (4) being on an average at least 99 percent by weight of the pellets in themixture leaving said retort zone; I

e. eventually passing the pellets from said separation zone to a pellet deposition burning zone; f. heating the pellets passed to said pellet deposition burning zone to an outlet temperature of between 1,000F and 1,500F by burning a combustible carbon-containing deposition on said pellets with a combustion supporting gas; and

g. thereafter, eventually passing at least a portion of the heated pellets from said pellet deposition burning zone to said retort zone.

2. The method according to claim 1 wherein prior to step (d)(l), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.

3. The method according to claim 1 wherein in steps (d)(1),(d)(2), and (d)(3), the inlet temperature ofthe elutriating gas is at a temperature above 750F.

4. The method according to claim 3 wherein prior to step (d)( 1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.

5. The method according to claim 1 wherein in step (d)( l) the percentage by weight of spent shale smaller than the smallest size pellets in the remainingmixture of pellets and spent shale after elutriation is less than 3 percent.

6. The method according to claim 5 wherein prior to step (d)(l), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.

7. The method according to claim 5 wherein in steps (d)( l (d)(2), and (d)(3), the inlet temperature ofthe elutriating gas is at a temperature above 750F.

8. The method according to claim 7 wherein prior to step (d)(l), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.

9. The method according to claim 1 wherein in step (d) the pellets recovered in steps (d)(2), (d)(3), and (d)(4) are passed to an elutriation zone and subjected to gas elutriation with a noncombustion supporting gas at an elutriation velocity below the elutriation velocity of the smallest size pellets but at least high enough to elutriate spent shale that is smaller than the smallest size pellets.

10. The method according to claim 9 wherein prior to step (d)( l the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.

1 l. The method according to claim 9 wherein in steps (d)( 1 (d)(2), and (d)(3), the inlet temperature of the elutriating gas is at a temperature above 750F.

12. The method according to claim 11 wherein prior to step (d)( l the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.

13. The method according to claim 1 wherein the pellets have a sphericity factor of at least 0.9.

14. The method according to claim 13 wherein prior. to step (d)( l the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.

15. The method according toclaim 13 wherein in steps (d)( l (d)(2), and (d)(3), the inlet temperature of the elutriating gas is at a temperature above 750F.

18. The method according toclaim 17 wherein prior to step (d)( 1 the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.

19. The method according to claim 17 wherein in steps (d)( l (d)(2), and (d)(3), the inlet temperature of the elutriating gas is at a temperature above 750F.

20. The method according to claim 19 wherein prior to step (d)( l the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.

21. The method according to claim 13 wherein in step (d) the pellets recovered in steps (d)(2), (d)(3), and (d)(4) are passed to an elutriation zone and subjected to gas elutriation with a noncombustion supporting gas at an elutriation velocity below the elutriation velocity of the smallest size pellets but is at least high enough to elutriate spent shale that is smaller than the smallest size pellets. I

22. The method according to claim 21 wherein prior to step (d)( l the mixture of pellets and spent shale is screened to screen out anyspent shale, mineral matter, and agglomerates larger than the pellets.

3 ,803 ,021 v v 16 23. The method according to claim 21 wherein in to step (d)( 1 the mixture of pellets and spent shale is Steps (dx 1 (d)(2)' and (CD6 the inlet temperature screened to screen out any spent shale, mineral matter,

of the elutriating gas is at a temperature above 750F.

and agglomerates larger than the pellets.

24. The method according to claim 23 wherein prior 5 

2. subjecting the mixture of remaining pellets and spent shale from the preceding gas elutriation stage to gas elutriation in an elutriation zone with a noncombustion gas at an elutriation velocity higher than the gas elutriation velocity of said preceding elutriation stage and at least high enough to elutriate the smallest size pellets in said mixture thereby also elutriating a portion of the spent shale larger than the elutriated pellets, the elutriated pellets and spent shale being separated as an overhead stream from said elutriation zone thereby leaving remaining particulate solids containing pellets of an average size larger than the average size of the pellets prior to said gas elutriation, screening said overhead stream to retain spent shale larger than the pellets in said overhead stream and to pass the pellets in said overhead stream, and recovering the screened pellets;
 2. The method according to claim 1 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.
 3. repeating step (2) until on an average at least 60 percent by weight of the spent shale from said retort zone having the same size as the pellets is separated from the pellets from said retort zone; and thereafter recovering the pellets remaining after the last elutriation stage, the total amount of pellets recovered in steps (2), (3), and (4) being on an average at least 99 percent by weight of the pellets in the mixture leaving said retort zone; e. eventually passing the pellets from said separation zone to a pellet deposition burning zone; f. heating the pellets passed to said pellet deposition burning zone to an outlet temperature of between 1,000*F and 1,500*F by burning a combustible carbon-containing deposition on said pellets with a combustion supporting gas; and g. thereafter, eventually passing at least a portion of the heated pellets from said pellet deposition burning zone to said retort zone.
 3. The method according to claim 1 wherein in steps (d)(1), (d)(2), and (d)(3), the inlet temperature of the elutriating gas is at a temperature above 750*F.
 4. The method according to claim 3 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.
 5. The method according to claim 1 wherein in step (d)(1) the percentage by weight of spent shale smaller than the smallest size pellets in the remaining mixture of pellets and spent shale after elutriation is less than 3 percent.
 6. The method according to claim 5 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.
 7. The method according to claim 5 wherein in steps (d)(1), (d)(2), and (d)(3), the inlet temperature of the elutriating gas is at a temperature above 750*F.
 8. The method according to claim 7 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.
 9. The method according to claim 1 wherein in step (d) the pellets recovered in steps (d)(2), (d)(3), and (d)(4) are passed to an elutriation zone and subjected to gas elutriation with a noncombustion supporting gas at an elutriation velocity below the elutriation velocity of the smallest size pellets but at least high enough to elutriate spent shale that is smaller than the smallest size pellets.
 10. The method according to claim 9 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pelletS.
 11. The method according to claim 9 wherein in steps (d)(1), (d)(2), and (d)(3), the inlet temperature of the elutriating gas is at a temperature above 750*F.
 12. The method according to claim 11 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.
 13. The method according to claim 1 wherein the pellets have a sphericity factor of at least 0.9.
 14. The method according to claim 13 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.
 15. The method according to claim 13 wherein in steps (d)(1), (d)(2), and (d)(3), the inlet temperature of the elutriating gas is at a temperature above 750*F.
 16. The method according to claim 15 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.
 17. The method according to claim 13 wherein in step (d)(1), the percentage by weight of spent shale smaller than the smallest size pellets in the remaining mixture of pellets and spent shale after elutriation is less than 3 percent.
 18. The method according to claim 17 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.
 19. The method according to claim 17 wherein in steps (d)(1), (d)(2), and (d)(3), the inlet temperature of the elutriating gas is at a temperature above 750*F.
 20. The method according to claim 19 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.
 21. The method according to claim 13 wherein in step (d) the pellets recovered in steps (d)(2), (d)(3), and (d)(4) are passed to an elutriation zone and subjected to gas elutriation with a noncombustion supporting gas at an elutriation velocity below the elutriation velocity of the smallest size pellets but is at least high enough to elutriate spent shale that is smaller than the smallest size pellets.
 22. The method according to claim 21 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets.
 23. The method according to claim 21 wherein in steps (d)(1), (d)(2), and (d)(3), the inlet temperature of the elutriating gas is at a temperature above 750*F.
 24. The method according to claim 23 wherein prior to step (d)(1), the mixture of pellets and spent shale is screened to screen out any spent shale, mineral matter, and agglomerates larger than the pellets. 